This invention relates to a digital filter and more particularly to a multiple-input digital filter.
Generally, when a digital signal is inputted to a digital device, it is converted to a stable digital signal by passing it through a filter for noise removal and then passing this filter output through a hysteresis circuit having two threshold values.
FIG. 1 is a block diagram showing a conventional digital filter.
In FIG. 1, a filtering unit 11 is an integrating filter using capacitance and resistance (C/R) which can cut the high frequency components of noise.
When a step signal containing noise X such as shown in FIG. 2(a), is inputted to the filtering unit 11, the filter output takes on the indistinct waveform shown in FIG. 2(b). If this filter output waveform is used in a digital circuit as it stands, the logical value becomes unstable in the region midway between the level which is recognisable as logical value "0" and the level recognisable as logical value "1", and therefore the operation of a digital circuit using these logical values becomes unstable.
If the filter output 0 is passed through a hysteresis circuit 12, it will change to the output Y taking the distinct form shown in FIG. 2(C) and the unstable region of logical values will be eliminated so that operation of the digital circuit can be stabilized. However, this digital filter can only process 1 digital input signal. Therefore, for digital inputs from multiple points, it is necessary to have the corresponding number of filters. Consequently, the number of parts required will increase and there will be problems of cost and space.
Moreover, the circuit characteristics of the hysteresis circuit 12 and CR integrating filtering unit 11 are determined by the elements used, and there is the problem that the characteristics for each input point will differ depending on the randomness of the characteristics of the element used.